In recent years there has been an increase in the installation of aerial tramway systems in which the passenger carrier units are selectively detachable and attachable to the tramway haul rope. Both gondola-based and chair-based aerial tramways are being installed in which the chair or gondola is provided with a haul rope gripping mechanism that can be opened and closed at the tramway terminals to allow the passenger carrier unit to be detached from the haul rope and slowed down for loading and unloading while the haul rope speed is maintained. Typical of the haul rope grip assemblies that enable detachment of the passenger carrier units is the grip assembly of my U.S. Pat. No. 4,403,552.
Prior to release of the gondola or chair from the haul rope, the passenger carrier unit is supported by a roller assembly carried by the gondola or chair hanger arm which roll on a guide rail extending immediately proximate the haul rope. Once the passenger carrier unit is detached from the haul rope, it usually will be decelerated at the terminal and then advanced at a relatively low speed along the rail through an unloading and loading zone. Thereafter the gondola or chair is accelerated on the guide rail to haul rope speed and attached to the haul rope on the fly. Attendants assist the tramway users in exiting and boarding the passenger carrier units at the unloading and loading zones while the units travel on the guide rail at a speed in the range of 20 to 80 feet per minute.
Aerial tramways with detachable passenger carrier units inherently have the problem of evenly spacing the detachable units along the length of the haul rope line. Prior aerial tramway systems have approached this problem by using distance based attachment systems. Each aerial tramway system will have a maximum permissible number of units which can be supported on the haul rope at any time. The maximum is often controlled by code in terms of a minimum time interval between passenger carrier units, typically a minimum of 9 seconds. For a maximum haul rope speed of 1000 feet per minute, for example, the minimum distance between chairs or gondolas permissible by a 9 second time interval is 150 feet. If a chair or gondola is attached to the haul rope at 150 foot intervals, the maximum number of passenger carrier units can be determined simply by dividing the overall length of the tramway haul rope by the minimum spacing distance.
Employing a distance-based uniform spacing system, however, is complicated by several factors. The maximum number of units which can be positioned on the haul rope is rarely an even number, and tramway operators often operate at less than maximum tramway capacity. An 8000 foot long lift, for example, would have 53.33 chairs or gondolas as a maximum number which can be attached to the haul hope on the uphill and the downhill sides of the haul rope, if the tram is operating at 1000 feet per minute with 9 second minimum intervals. The tramway operator may decide, for various reasons, that only 45 chairs or gondolas will be used instead of 53.33. To evenly space the passenger carrier units on each side of the rope, the gondola attachment system must attach a chair every 177.78 feet.
At least three distance-based gondola attachment systems can be found in the industry. Virtually all of the prior systems include a section of storage conveyor rail upon which detached units are slowly advanced, often by gravity, to an electric launching clutch which releases the units to the launching system. The launching system includes an accelerator plus a grip actuating mechanism to cause the grip to close around the moving haul rope once the chair or gondola reaches the haul rope velocity.
In the first system, a haul rope odometer measures the length of cable passing before it and releases the electric launching clutch periodically in accordance with the spacing desired between units. The launching clutch holds up units on the storage conveyor rail until sufficient cable has passed through the odometer, and then the clutch is released. If there are no units in the storage section, the odometer opens the clutch, and the next unit arriving at the clutch is free to be launched.
In a second system a sensor, such as a wand or arm, engages a portion of the passenger carrier unit, such as the hanger arm, and the sensor is positioned downstream of the electric clutch and the launching assembly. As the preceeding unit hanger arm moves past the downstream sensor, the electric clutch is signalled to allow release and launching of the next unit. This approach is satisfactory for a fixed spacing, but when the spacing between units is varied, the sensor needs to be repositioned or an adjustment made through a timing delay.
A third system which is found in the prior art is a storage conveyor in which the units are engaged, usually at their hanger arms, by forks or arms which are carried at predetermined fixed intervals along a drive chain. The drive chain advances the gondolas or chairs around the storage conveyor rail to the launching area in a fixed relationship with the rate of movement of the chain being slaved to the rate of movement of the haul rope. Thus, the chain may advance at a linear rate which is much less than the haul rope, but it is slaved or controlled so that when the haul rope speed slows down, the chain advance rate slows to maintain the same relative proportion between the two.
This fixed-distance spacing conveyor has been found to have several disadvantages. First, if a unit should arrive a little late and miss the space between the arms or forks which pick up the units, there will be a gap of one complete unit along the haul rope. There will be a plus or minus 100% spacing error. This problem tends to perpetuate itself on subsequent cycles of the units. Additionally, if the tramway operator wishes to employ something less than the maximum capacity of the fixed-distance spacing conveyor, considerable problems are encountered. If it is desired to send out half the maximum number of chairs, it is a simple matter to simply remove every other chair from the fixed-distance spacing conveyor. Similarly, even spacing can be achieved for other percentages, for example 33.33%, but one cannot simply remove chairs from a fixed-distance spacing conveyor and get an even spacing along the haul rope for 80% of maximum capacity, for example.
Even when chairs are removed from the fixed-distance spacing conveyor to reduce the capacity below maximum, the result is there are fewer chairs at each of the terminals in the spacing conveyor. This in turn means that the time to load and unload is reduced, making the loading and unloading procedure more hazardous. Finally, prior art fixed-distance spacing conveyors with intervals defined by arms on a drive chain inherently require that the passenger carrier units be driven at the same speed over the entire length of the drive chain. It is not possible, therefore, to accelerate a unit and then decelerate or stop it to enable loading and unloading without accelerating, decelerating and stopping all of the units.